Heretofore, in this field, a powering antenna has often been provided with the interrogator in addition to the communicating antenna. This powering antenna is provided for the powering of a transponder not having its own internal power supply. The powering antenna may be a high Q-factor antenna to effect maximum power transfer to the transponder. Because of the flexibility afforded by having a separate power antenna in the interrogator, which may be optimized for power transfer to the transponder, the transponder antenna may be optimized for communicating with the interrogator's communicating antenna. An example of this type of system is given in U.S. Pat. No. 4,550,444 by Uebel and in U.S. Pat. No. 4,912,471 by Tyburski et al. In such systems the RF coupling can be separately designed for the power transfer link and the communications link. The disadvantage in such a system is the inherently greater cost and size involved in a transponder having two separate circuits for powering and communicating. Another known technique allowing for somewhat optimized powering and communication links is to provide separate transmit and receive antennas in the transponder. In this way the downlink (i.e., the communication link from the interrogator to the transponder) can be designed for efficient power transfer. Because the transponder is desirably compact and power-efficient, the uplink (i.e., the communication link from the transponder to the interrogator) can be optimized to the transponder's transmit antenna. Communication can still be effectively carried out over the downlink because of the lesser need for power and cost efficiency in the interrogator transmitter design. An example of this type of system can be found in U.S. Pat. No. 3,713,148 by Cardullo et al. As before, the disadvantage in such a system is the inherently greater cost and size involved in a transponder having two separate circuits, this time for receiving and transmitting.
Yet another known technique is described in U.S. Pat. No. 5,053,774 by Schuermann, et al. In this technique, communication and power transfer is preferably accomplished by a single resonant circuit in each of the interrogator and the transponder thereby minimizing cost, size and power efficiency. The resonant circuit in each of the interrogator and the transponder is used for both communication and power transfer in this prior art. For optimal power transfer the prior art device uses highly tuned, high Q-factor resonant circuits. Because in this technique the same resonant circuit is used in each of the interrogator and transponder for powering and/or bi-directional data communication, interrogators that are operating in close proximity to each other must be synchronized so that the RF power transmission phases, or powering signals do not interfere with the communication signals. In the case of interrogators and transponders that communicate bi-directionally, the data transmissions from the interrogators must also be synchronized so that a data transmission from one interrogator to a transponder does not interfere with the generally lower amplitude data transmission from a transponder to another interrogator.